How to use base href?

This is the first time I've tackled complex subdirectories, and I'm trying to figure out how to get subdirectory pages to access css and image folders located in the main directory. What am I missing? When I put just "css/main.css" as a link in a child page, the browser looks for the css WITHIN the current (child) folder. Do I need to enter the full URL for those files? Suck.

I thought that declaring the base href at the top of all pages might work (that is, using the base href of the main index page on ALL the subpages so that all links within those documents complete relative to the main URL rather than the current URL).

However, I see it warned here (http://chrisjdavis.org/base-href) that base href is intended to declare the full current page ID, unique to each document. Which throws me back to the original problem of shorthand links being perceived by the browser as children to the current page. Grrr!

This is all exacerbated by the fact that I don't have access to the client's ftp yet, so I can't use the correct URLs or anything, but couch it all in folders on my own site.